The exemplary embodiments relate to the art of digital imaging. It finds particular application in macro uniformity corrections for non-uniformities in a raster output scanning (ROS) printing system and will be described with particular reference thereto. It will be appreciated, however, that the disclosure is also amenable to other like applications.
Macro non-uniformity levels have existed in raster scan image output terminals (IOTs) (e.g., xerographic printers) for some time and are a concern for most marking processes. Even small non-uniformity level errors in raster scan IOTs give rise to visually objectionable banding in halftone outputs (e.g., image macro non-uniformity streak artifacts). Such errors typically arise in raster scan image output terminals (IOTs) due to variations in ROS spot size across the field (which is constant in time (print to print)), donor-roll once-around, HSD wire hysteresis, laser diode variations, LED bar power variation, ROS scan line non-uniformity, photoreceptor belt sensitivity variations, and/or ROS velocity non-uniformity. Significantly, many variations occur only in the fast scan (e.g., X) or slow scan (e.g., Y) directions, and they do not interact to first order. Therefore, a correction made in one direction has a negligible effect on artifacts in the other direction. Other printing technologies (e.g. thermal inkjet and acoustical ink printing) also have artifacts that occur in a regular, predictable manner in one or both directions and fall within the scope of this discussion.
Although techniques have been proposed to eliminate such non-uniformity errors by making physical systems more uniform, it is too expensive to control or limit the error to an acceptable level, below which the error will not be detected by the unaided eye. Fixes have been attempted in the marking process, but not enough latitude exists to fully solve the problem. For problem sources such as LED non-uniformity, the correction is sometimes addressed with current control or pulse width control. However, none of the solutions discussed above implements a technique based in digital electronics. With the cost of computing rapidly decreasing, such digital electronics based solutions are becoming more attractive.
The exemplary embodiments provide a new and improved method which overcomes the above-referenced problems and others. The exemplary embodiments relate to a method for sensing print defects in electrostatically formed images. It is to be appreciated that the exemplary embodiments are also amenable to other like applications.
Various apparatuses for recording images on sheets have heretofore been put into practical use. When the subsystems of an electrophotographic or similar image forming system operate under suboptimal conditions, a lack of print uniformity may result. Streaks can arise from a non-uniform LED imager, contamination of the high voltage elements in a charger, scratches in the photoreceptor surface, etc.
In a uniform patch of gray, streaks and bands may appear as a variation in the gray level. In general, “gray” refers to the intensity value of any single color separation layer, whether the toner is black, cyan, magenta, yellow, or some other color. One method of reducing such streaks is to design and manufacture the critical parameters of the marking engine subsystems to tight specifications. Often though, such precision manufacturing will prove to be cost prohibitive.
The streaks that can arise from the different subsystems can be prevented by modifying the image or actuating another subsystem to counteract the streak. To counteract streaks that arise, their size and magnitude must be sensed and measured with high precision. One of the image quality attributes of high quality printers is spatial uniformity in the cross process direction. In order to monitor the spatial uniformity, an accurate image processing technique is required to measure the uniformity. The image processing algorithms heretofore known, for detecting or sensing defects, fail or give erroneous results. Making high precision measurements of the streak's magnitude and size is limited by distortions that occur during the printing of the image and/or scanning of the image. The distortions may not be objectionable in viewing typical images, but they may be of a magnitude that prevents an accurate measurement of the degree of streaking. Examples of printing and/or scanning defects include process and cross process position waviness, image rotation, process direction expansion of the image, image deletions, background toner, and scanner induced distortion of the image.
A tone reproduction curve (TRC) may be measured by printing patches of different bitmap area coverage. In some digital image processing applications, the reflectivity of a patch of gray is measured with a toner area coverage sensor. The manner of operation of the fixed position sensor is described in U.S. Pat. No. 4,553,033, which is incorporated herein by reference in its entirety. Toner area coverage sensors are typically designed with an illumination beam much larger than the halftone screen dimension. This large beam does not provide the resolution for the toner area coverage sensor to be useful as a sensor for the narrow streaks that may occur for poorly performing subsystems.
U.S. Pat. No. 6,760,056 by Klassen et. al, incorporated herein by reference in its entirety, discloses one exemplary embodiment of a method for compensating for streaks introducing a separate tone reproduction curve for each pixel column in the cross process direction. A compensation pattern is printed and then scanned to first measure the ideal tone reproduction curve and then detects and measure streaks. The tone reproduction curves for the pixel columns associated with the streak are then modified to compensate for the streak.